This invention relates to a radio receiver, and more particularly to an automatic frequency scanning radio receiver.
As automatic frequency scanning radio receivers that automatically scan a plurality of channels within a predetermined radio frequency band to automatically select a radio frequency broadcast signal on one channel, there are known those receivers which employ a phase-locked loop frequency synthesizer as a local oscillator circuit. Such phase-locked loop frequency synthesizer comprises a voltage-controlled oscillator (VCO), a divide-by-N counter for dividing the output frequency of VCO, a phase comparator for detecting the phase difference between an output signal of the counter and a reference frequency signal, and a loop filter coupling a DC voltage having a magnitude corresponding to the phase difference between the input signals of the phase comparator to VCO as a control signal. For the divide-by-N counter, there is used a programmable counter capable of designating a dividing factor N by means of a program input signal or station select signal, whereby the output frequency of the frequency synthesizer or local oscillator frequency may be specified. In other words, when a specific number (dividing factor) is designated by the program input signal, a channel corresponding to the designated number is selected to receive a broadcast signal on the channel.
When the dividing factor N is automatically changed in integral steps by the program input signal, the frequency synthesizer has its output frequency automatically changed in steps at predetermined frequency spaces to scan channels automatically. As the program input signal for the automatic scanning, there are usually used outputs of scan counters for counting clock pulses with a sufficiently low frequency.
In the prior art automatic frequency scanning radio receivers as described above, the received frequency (dividing factor) is displayed on a display device during the automatic scanning. When the displayed value attains a desired broadcasting frequency, a scanning stop switch is operated to interrupt the automatic scanning. Since the output frequency of the synthesizer varies constantly during the automatic scanning, it is difficult to stop the scanning exactly at the desired frequency. Therefore, a fine tuning adjustment is essential after the automatic scanning is stopped. Moreover, in the conventional radio receivers, there is provided a one-step scanning switch for performing a step-by-step manual scanning. To perform the manual scanning by means of the one-step scanning switch the above-mentioned automatic scannning stop switch must be operated to interrupt the automatic scanning once. That is, these conventional automatic scanning radio receivers are subject to a disadvantage; the operation for the automatic selection of station and the like are troublesome.